Liquid silane-based compositions and methods for producing silicon-based materials

ABSTRACT

Described herein are synthesis schemes and methods for producing silicon based nanostructures and materials, including compositions and methods for synthesis of silicon-based nanowires and composites from three-component and four-component liquid silane/polymer inks. Materials and methods for producing silicon based micro and nanofibers that can be used in a variety of applications including material composites, electronic devices, sensors, photodetectors, batteries, ultracapacitors, and photosensitive substrates, and the like.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. patent application Ser. No.13/645,551 filed on Oct. 5, 2012, incorporated herein by reference inits entirety, which is a 35 U.S.C. §111(a) continuation of PCTinternational application number PCT/US2011/031478 filed on Apr. 6,2011, incorporated herein by reference in its entirety, which claimspriority to, and the benefit of, U.S. provisional patent applicationSer. No. 61/321,338 filed on Apr. 6, 2010, incorporated herein byreference in its entirety. Priority is claimed to each of the foregoingapplications.

The above-referenced PCT international application was published as PCTInternational Publication No. WO 2011/127218 on Oct. 13, 2011 andrepublished on Feb. 2, 2012, and is incorporated herein by reference inits entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with Government support under Grant NumberEPS-0447679 awarded by North Dakota EPSCoR/National Science Foundationand under agreement Number H94003-09-2-0905 awarded by the DoD DefenseMicroelectronics Activity (DMEA). The Government has certain rights inthe invention.

INCORPORATION-BY-REFERENCE OF COMPUTER PROGRAM APPENDIX

Not Applicable

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention pertains generally to synthesis schemes and methods forproducing silicon based nanostructures and materials, and moreparticularly to compositions and methods for synthesis of silicon-basednanowires and composites from three-component and four-component liquidsilane/polymer inks.

2. Description of Related Art

Future generation electronics will feature components that aremanufactured by continuous processing. Two-dimensional webs that serveas the substrate in roll-to-roll manufacturing are subjected to additiveprocesses where various materials are deposited and then transformed togive functioning circuit components. The beneficial electrical andelectrochemical properties of silicon have been demonstrated inintegrated circuits, solar cells and battery electrodes. Such materialsare typically produced by chemical vapor deposition or by etching a Siwafer and these processes are not amendable to continuous manufacturing.

For example, there is increasing interest in replacing carbon-basedmaterials with silicon or silicon-based compounds as anodes innext-generation lithium ion batteries (LIBs). Silicon has a theoreticalcapacity of approximately 4200 mAh/g, which is more than ten timesgreater than the 372 mAh/g capacity of conventional graphite anodematerials. Therefore, Si-based anodes could increase the energy densityof lithium ion batteries significantly.

However, fully lithiated silicon (Li₂₂Si₅) undergoes a >300% volumeexpansion during the lithiation and delithiation process which leads tomechanical failure of the silicon structure within a few cycles leadingto a significant and permanent loss of capacity. A number of approachestoward the development of silicon-containing anodes have been attempted.One approach was the use of a homogeneous dispersion of siliconparticles within a suitable matrix to give composites that have improvedmechanical stability and electrical conductivity versus pure silicon. Ithas been shown that silicon nanowires or fibers are able to accommodatethe expansion that occurs during cycling. However, significant numbersof Si-nanowires (SiNWs) are needed for practical anode applications.

A Vapor Induced Solid-Liquid-Solid (VI-SLS) route to SiNWs has beenproposed that uses bulk silicon powders thus offering the possibility ofscalable and cost-effective mass manufacture without the need for alocalized catalyst on a substrate. The VI-SLS process, however, iscomplicated by high process temperatures that tend toward the formationof carbide and oxide phases that limit electrochemical capacity and ratecapabilities.

Another approach to the production of silicon nanowires is throughelectrospinning where the electrospun polymer fiber serves only as atemplate for the growth of silicon coatings by hot-wire chemical vapordeposition (CVD) or plasma enhanced CVD (PECVD). While these routes doallow the growth of a-Si nanowires with hollow cores, hot-wire and PECVDsuffer from poor precursor utilization and traditionally slow growthrates.

Accordingly, there is a need for an apparatus and method for reliablyproducing silicon based nanowires and films that are inexpensive andamenable to continuous roll-to-roll operation. The present inventionsatisfies these needs as well as others and is generally an improvementover the art.

BRIEF SUMMARY OF THE INVENTION

The present invention is directed to materials and methods for producingsilicon based micro and nanofibers that can be used in a variety ofapplications including material composites, electronic devices, sensors,photodetectors, batteries, ultracapacitors, and photosensitivesubstrates and the like.

Liquid silanes have been considered as precursors in direct-writefabrication of printed electronics. Cyclohexasilane (Si₆H₁₂), forexample, can be transformed into solid polydihydrosilane (SiH₂)_(n) bythermal treatment or light activation via radical polymerization.Additional thermolysis causes evolution of H₂ (g) giving a-Si:H at ˜350°C. and crystalline silicon at ˜750° C.

Marked microstructural changes, however, are associated with thisthermolytic transformation. The thermal conversion of Si₆H₁₂-derivedfilms and/or (SiH₂)_(n) into a-Si occurs with marked shrinkage around290° C. and it appears to be related to the evolution of SiH₂ and SiH₃fragments. This phenomenon may limit electrical transport owing tomicrocracking within these thin films. This shrinkage does not lead tocracking when films are less than a thickness of ˜200 nm. Theelectrospinning methods of the present invention appear to manage thestress, in part, by reducing the dimensionality from 2D films to 1Dwires.

Electrospinning, according to the invention, is a viable method forutilizing liquid cyclosilanes (i.e., Si_(n)H_(2n)) and linear orbranched silanes (i.e., Si_(n)H_(2n+2)) in the fabrication of electronicmaterials as these monomers are transformed directly into a useful form(i.e., a nanowire) prior to the formation of the insoluble (SiH₂)_(n)network polymer. The lateral cohesive stresses that promote cracking inthe aforementioned 2D thin films are well managed in 1D wires whereradial shrinkage does not lead to the observed deleteriousmicrostructural changes of larger silicon structures.

Electrospinning is a continuous nanofabrication technique based on theprinciple of electrohydrodynamics, and it is capable of producingnanowires of synthetic and natural polymers, ceramics, carbon, andsemiconductor materials with the diameter in the range of 1 to 2000 nm.While the Taylor cone instability associated with electrospinning washistorically used for nozzle-based systems, the surface instability ofthin polymer-in-solution films in the presence of an electric fieldenabled the development of needleless electrospinning whereby numerousjets spin coincidently allowing a continuous, roll-to-roll manufacturingprocess. Additionally, continuous needleless electrospinning thatutilizes a rotating cone as the spinneret has been demonstrated withproduction throughput of up to 10 g/minute.

This is in stark contrast to the two common silicon nanowire preparationmethods known in the art where the ability to scale up appears to belimited by wafer size (i.e., when forming Si nanowires via waferetching) or a growth temperature of ˜363° C. (i.e., Au-Si eutectic invapor-liquid-solid growth). In each instance, the transition to acontinuous roll-to-roll manufacturing process is not straightforward andmay not be possible.

It has been observed that the liquid silane monomers that are used inthe invention are relatively unaffected by the high-voltageelectrospinning process and remains associated with the polymericcarrier (i.e., poly(methyl methacrylate (PMMA) or polypropylenecarbonate/polycyclohexene carbonate (QPAC100™, Empower Materials)) uponevaporation of the toluene or other solvent. Light- or heat-inducedradical polymerization of the Si₆H₁₂ gives a viscous polydihydrosilanedeposit that assumes a geometry that is related to the structure of thecopolymer. The structure of the silicon nanowires prepared fromSi₆H₁₂/polymer carrier in toluene mixtures appears to be governed by thephysics of the copolymer mixtures. For example, the SEM data shows thata fibrous structure is formed after treating an electrospun compositeformed from a 1.0:2.6 wt % ratio of Si₆H₁₂/PMMA in toluene ink. Thisstructure appears to be related to wetting of the polymer by the liquidsilane after solvent evaporation. By way of comparison, thermolysis ofthe composite formed by electrospinning a 1.0:2.0 wt % ratio ofSi₆H₁₂/QPAC100 in toluene precursor gives a porous wire where it appearsthe liquid silane and polymer carrier exist as a microemulsion and phaseseparate after solvent evaporation.

It has also been observed that electrospinning three-componentSi₆H₁₂/polymer inks gives products where the active silicon agent formsafter the precursor is transformed to nanosized material. The approachoffers the ability to tailor chemical composition of Si wires byadjusting precursor chemistries to give electrospun composites thatpossess targeted conductivities (electrical, thermal and ionic) andmaintain structural stability throughout a lifetime of charge/dischargecycles. Barring any undesirable chemical reactivity with Si—Si or Si—Hbonds, particles of carbon, metals and solid electrolytes can beintroduced into liquid silane-based electrospinning inks using standarddispersion chemistry. Because the spun wires convert to amorphoussilicon at relatively low temperature, formation of excessive surfaceoxide and carbide phases can be avoided, which otherwise negativelyaffect capacity and rate capabilities. It is important to note thatother routes to Si wires yield crystalline products that becomeamorphous after lithium intercalation in LIBs.

The three-component and four-component inks that are disclosed areparticularly useful with electrospinning procedures and the formation ofmicro and nanofibers are used as an illustration. However, the inks canalso be used with other deposition techniques such as thin filmdeposition techniques. In addition, single or coaxial nozzle formationof nanofibers is used to illustrate the methods. However, it will beunderstood that the inks and methods of the invention are appropriatefor any electrospinning technique including use with devices that havemultiple nozzles, drums or films.

By way of example, and not of limitation, a preferred method for makingsilicon-containing wires with a three-component ink generally comprisesthe steps of: (a) combining a liquid silane of the formula Si_(n)H_(2n)or Si_(n)H_(2n+2), a polymer and a solvent to form a viscous solution;(b) expelling the solution from a source while exposing the stream ofviscous solution to a high electric field resulting in the formation ofcontinuous fibers that are deposited onto a substrate; and (c)transforming the deposited fibers, normally with thermal processing.

In another embodiment of the invention, a preferred method for makingsilicon-containing wires with a four-component ink generally comprises:(a) combining a liquid silane of the formula Si_(n)H_(2n) orSi_(n)H_(2n+2), a polymer, a solid phase and a solvent to form a viscoussolution; (b) expelling the viscous solution and exposing the viscoussolution to a high electric field whereby continuous fibers form fromthe solution and are deposited onto a substrate; and (c) transformingthe electrospun deposit.

The solid phase components are preferably particulates of many differenttypes such as metal spheres, silicon nanowires, carbon particulatesincluding nanotubes, as well as dopants, and metal reagents. Forexample, metal silicide wires can be formed with addition of metalreagents.

The polymers are preferably either a an acrylate such as poly(methylmethacrylate) or a polycarbonate. The preferred solvents are toluene,xylene, cyclooctane, 1,2,4-trichlorobenzene, dichloromethane or mixturesthereof.

The substrate is preferably a metal foil. However, the substrate mayalso be a carbon fiber matte, metal web or rotating mandrel.

Transformation of the deposit is preferably by thermal treatment orlight activation via radical polymerization. Transformation of thedeposited nanofibers can take place at any time or location and need nottake place on the substrate.

In certain embodiments, the methods for producing silicon basednanofibers may further include the step of coating the fibers with anelectrically conductive material. The preferred coating is a coherent,ion conductive coating of carbon such as graphite, C black, graphene, KBcarbon or carbon nanotubes. The coating of the fibers is preferablyapplied by chemical vapor deposition or solution deposition.

The silicon-based materials and nanofibers that are produced by thethree- and four-component inks can be used in a variety of applicationsincluding as an active component in other composite materials. Forexample, electrically-conducting silicon composite electrodes can beproduced with a three-component ink according to the invention by (a)combining a liquid silane of the formula Si_(n)H_(2n), orSi_(n)H_(2n+2), a polymer and a solvent to form a viscous solution; (b)expelling the viscous solution into the presence of a high electricfield where continuous fibers are formed and deposited onto a substrate;(c) transforming the deposit into a material that contains a polysilane,an amorphous silicon and/or a crystalline silicon fraction with orwithout a binder; (d) forming a coherent, conductive coating on theexternal porosity of the silicon-containing fraction and (e) binding thematerial with one or more binders. The preferred binders includepoly(vinylidene fluoride-co-hexafluoropropylene) or sodiumcarboxymethylcellulose or an elastic carbon such as KB carbon. Somebinders can be thermally decomposable.

Another example of a composite material that can be produced is anelectrically-conducting photoactive silicon-composite electrode materialusing a four-component ink. This material can be produced by (a)combining a liquid silane of the formula Si_(n)H_(2n) or Si_(n)H_(2n+2),a polymer, a photoactive solid phase and a solvent to form a viscousmixture; (b) expelling the viscous mixture into the presence of a highelectric field where continuous fibers of the mixture are formed anddeposited onto a substrate; (c) transforming the deposit into a materialthat contains an amorphous silicon and/or a crystalline silicon fractionand a photoactive phase; and binding the transformed material with abinder. The preferred photoactive phase can be a carbon fullerene, acarbon nanotube, a quantum dot of CdSe, PbS, Si or Ge, a core-shellquantum dot of ZnSe/CdSe or Si/Ge.

Accordingly, an aspect of the invention is to provide three-component orfour-component silane inks that can be used in the formation of siliconbased films and nanofibers and composite materials.

Another aspect of the invention is to provide methods for producingpolysilane nanowires and materials.

Another aspect of the invention is to provide a method for continuousproduction of nanofiber strands and coated nanofiber strands.

A further aspect of the invention is to provide silicon based fibersthat can be used as a component in a variety of composite materials suchas electrode composites.

Further aspects of the invention will be brought out in the followingportions of the specification, wherein the detailed description is forthe purpose of fully disclosing preferred embodiments of the inventionwithout placing limitations thereon.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The invention will be more fully understood by reference to thefollowing drawings which are for illustrative purposes only:

FIG. 1 is a flow diagram of a method of forming silicon based nanofibersfrom a three-component ink according to one embodiment of the invention.

FIG. 2 is a flow diagram of a method of forming silicon based nanofibersfrom a four-component ink according to another embodiment of theinvention.

FIG. 3 is a flow diagram of a method for producing an electrode materialfrom carbon coated silicon nanofibers formed according to one embodimentof the invention.

FIG. 4 is a schematic diagram of the processing of cyclohexasilane andPMMA in toluene, a three-component ink, to produce transformednanofibers.

FIG. 5 is a schematic diagram of the processing of cyclohexasilane andQPAC100 in toluene, a three-component ink, to produce transformednanofibers.

FIG. 6 shows Raman spectra of electrospun four-component samples afterheat treatment at 550° C. for one hour and laser crystallization forCdSe, C black, graphite, Ag, amphiphilic invertible micelle (AIP), BBr₃and PBr₃.

DETAILED DESCRIPTION OF THE INVENTION

Referring more specifically to the drawings, for illustrative purposesone embodiment of the present invention is depicted in the methodsgenerally shown in FIG. 1 through FIG. 6. It will be appreciated thatthe methods may vary as to the specific steps and sequence and theapparatus may vary as to structural details, without departing from thebasic concepts as disclosed herein. The steps depicted and/or used inmethods herein may be performed in a different order than as depicted inthe figures or stated. The steps are merely exemplary of the order thesesteps may occur. The steps may occur in any order that is desired, suchthat it still performs the goals of the claimed invention.

The present invention provides methods for producing silicon containingnanowire/fiber composites and thin films that are produced from liquidsilane inks by electrospinning as an illustration of an adaptation ofthe invention. Nanowire products from three-component and four-componentliquid silane based “ink” compositions are produced and characterized todemonstrate the methods. The exemplary nanowires that are produced bythe methods can be used as a component of other material compositionssuch as an anode for a lithium ion battery.

Turning now to FIG. 1, the steps according to a preferred embodiment 10of the present method for producing a silicon based nanowire materialusing three-component liquid silane inks with an optional conductivecoating is illustrated. At block 12, a solution of a liquid silane, apolymer and a solvent is provided. The resulting viscous solutionpreferably has a viscosity of approximately 100 cP to approximately10,000 cP for electrospinning procedures.

The preferred liquid silane has the formula Si_(n)H_(2n), where n=3, 4,5, 6, 7 or 8. Linear and branched liquid silanes of the formulaSi_(n)H_(2n+2), where n=3, 4, 5, 6, 7 or 8 may also be used. Mixtures ofone or more of these silanes may also be used.

Cyclohexasilane (Si₆H₁₂) is a particularly preferred cyclosilane. LiquidSi₆H₁₂ is preferably synthesized by reduction of a chlorinated saltprepared from trichlorosilane (HSiCl₃). Cyclohexasilane is a highmelting point liquid (18° C.) that is stable toward reduced-pressuredistillation as well as ambient light. Si₆H₁₂ has been shown to bestable to room temperature fluorescent light for days and it can bestored for months in the solid state without marked degradation. Si₆H₁₂is stable toward ultrasonic atomization and has been used as a precursorin collimated aerosol beam direct write deposition of a-Si lines. Inaddition, Si₆H₁₂ is stable when subjected to high voltage processing andelectrospinning procedures to yield a-Si nanowires that may findapplication as anodes in lithium ion batteries and other materials.

In the embodiment shown in FIG. 1, Si₆H₁₂ undergoes ring openingpolymerization under heat or prolonged exposure to laser light withadditional thermal treatment transforming the solid polydihydrosilane(SiH₂)_(n) into amorphous silicon first and then crystalline siliconmaterial. Specifically, Si₆H₁₂ can be transformed into solidpolydihydrosilane (SiH₂)_(n) by thermal treatment or light activationvia radical polymerization. Additional thermolysis causes evolution ofH₂ (g) giving a-Si:H at ˜350° C. and crystalline silicon at ˜850° C.

In another preferred embodiment, the liquid silane is cyclopentasilane,cyclohexasilane and/or 1-silylcyclopentasilane corresponding toSi_(n)H_(2n) where n=5 or 6.

The preferred polymer is poly(methyl methacrylate). However, apolycarbonate such as polypropylene carbonate/polycyclohexene carbonateor poly(vinylidene fluoride-co-hexafluoropropylene) and polyvinylbutryal may also be used in the embodiment shown at block 12 of FIG. 1.

In one embodiment, the percentage of silane to organic polymer in theviscous solution is kept within the range of approximately 5% to 20%silane, with the range of 10% to 16% silane preferred.

The preferred solvents at block 12 include toluene, xylene, cyclooctane,1,2,4-trichlorobenzene, and dichloromethane or mixtures thereof.However, although these solvents are preferred, it will be understoodthat other solvents may be selected based on the polymers and thesilanes that are employed.

At block 14, the viscous solution produced at block 12 is expelled froma nozzle or drawn from a film and exposed to a high electric field andcontinuous fibers arising from the solution are formed and depositedonto a substrate.

In one embodiment of the method, the high-voltage environment is formedby applying a d.c. bias from the point where the solution is expelledfrom a nozzle to the collecting substrate. The voltage used for theelectrospinning process normally ranges from approximately 5000V toapproximately 20,000V with approximately 7000V to 11,000V typicallyused. In a preferred embodiment, a direct current bias that is greaterthan approximately 2 kV is applied across a gap of 10 cm in a nitrogenenvironment.

The electrospinning apparatus can also have a nozzle with an innerannulus and an outer annulus. In this configuration, liquid silane isexpelled through the inner annulus of a coaxial delivery tube whileviscous polymer solution is expelled through the outer annulus and bothfluids are exposed to a high electric field resulting in the continuousformation of fibers that are deposited onto a substrate.

In one preferred configuration, the liquid silane that is directedthrough the inner annulus is Si₆H₁₂ cyclohexasilane, Si₆H₁₂,1-silyl-cyclopentasilane or Si₅H₁₀ cyclopentasilane and the solutionflowing through the outer annulus is polyacrylonitrile indimethylformamide.

The strand of nanofiber material that is formed from solution expelledfrom the nozzle in a high electric field at block 30 is deposited andcollected on a substrate at block 40. In the embodiment shown in FIG. 1,the substrate consists of a metallic foil such as copper foil oraluminum foil. In one configuration, the substrate includes conductivemetallic portions and insulating portions and the silicon-containingwires that are produced span the insulating portions of the substrate.In another embodiment, the substrate is a conducting carbon fiber matteincluding a carbon fiber matte constructed of carbon nanotubes. Thesubstrate may also be a rotating mandrel or a moving metal web of foilsuch as copper foil.

At block 50 the deposited and collected nanowires are transformed usingthermal processing or laser processing. With cyclohexasilane basedsolutions, for example, the deposit can be transformed using thermalprocessing at temperatures ranging from approximately 150° C. to 300° C.to produce polysilane-containing materials. The deposit can also betransformed using thermal processing at temperatures ranging from about300° C. to about 850° C., producing amorphous silicon-containingmaterials. The deposit from block 40 can be transformed using thermalprocessing at temperatures from ˜850° C. to 1414° C. producingcrystalline silicon-containing materials. As an illustration, thethermal treatment of cyclohexasilane and polymer solvent expelledthrough a coaxial nozzle consists of 350° C. under N₂ for one hourfollowed by 350° C. in air for one hour followed by 800° C. in N₂ forone hour. The deposit can also be transformed using laser processing toproduce crystalline silicon-containing materials.

Optionally, at block 60, the transformed fibers can be coated with acoherent, conductive coating and the coated transformed fibers can beused as a component of composite materials such as an anode material fora lithium ion battery, for example.

In one embodiment, the conductive coating is deposited by chemical vapordeposition using argon/acetylene, hydrogen/methane or nitrogen/methaneas precursor gases. In another embodiment, the coherent, conductivecoating is deposited by solution deposition. For example, the solutiondeposition may employ a dispersion of conducting carbon milled togetherwith the silicon-containing fraction in solvent. The conductive carboncan be graphite, carbon black, graphene, or carbon nanotubes in thisembodiment.

Referring now to FIG. 2, the steps according to a preferred embodiment100 of the present method for producing a silicon-based nanowirematerial using four-component liquid silane inks with an optionalconductive coating is illustrated. Four-component inks, according to theinvention, may have essentially the same components as thethree-component inks described herein with the addition of a solidphase. The solid phase may be a particulate, photoactive or a reactivecompound. Processing of the four-component inks is typically the same asthe processing of the three-component inks.

At block 110, a viscous solution is formed by combining a liquid silanepreferably of the formula Si_(n)H_(2n), a polymer, a solid phase and asolvent. As with the three-component inks, the components may becombined sequentially in any order or by pairs.

The preferred liquid silane has the formula Si_(n)H_(2n), where n=3, 4,5, 6, 7 or 8. Linear and branched liquid silanes of the formulaSi_(n)H_(2n+2), where n=3, 4, 5, 6, 7 or 8 may also be used. Mixtures ofone or more of these silanes may also be used.

The preferred polymer is poly(methyl methacrylate) or a polycarbonate inthe embodiment shown at block 110 of FIG. 2. The preferred solvents atblock 110 of FIG. 2 include toluene, xylene, cyclooctane,1,2,4-trichlorobenzene, and dichloromethane or mixtures thereof.However, although these polymers and solvents are preferred, it will beunderstood that other polymers and solvents may be selected based on thepolymers, the solid phases and the silanes that are employed.

One or more solid phase components can be part of the ink mixtureprovided at block 110 of FIG. 2. For example, the solid phase cancomprise a plurality of metallic particles, preferably nanoscaleparticles, which may be spherical or have a high aspect ratio. In oneembodiment, the metallic particles are made of a metal such as Al, Au,Ag, Cu, In—Sn—O, fluorine-doped tin oxide, or a metal alloy. In anotherembodiment, the particles may be made from graphite, carbon black, orgraphene. The metallic particles may also be composed of wires or tubesof suitable dimensions such as carbon nanotubes or silicon nanowires.

In other embodiments, the solid phase contains elements that are knownto substitutionally-dope silicon such as boron, phosphorous, arsenic orantimony containing compounds. The solid phase component can also besemiconducting particles formed from materials such as carbon nanotubes,CdSe, CdTe, PbS, PbSe, ZnO or Si.

The solid phase component can also include polydihydrosilane—(SiH₂)_(n)—, formed by UV-irradiation of Si_(n)H_(2n) (n=5, 6)corresponding to cyclopentasilane, cyclohexasilane and/or1-silylcyclopentasilane.

In another embodiment, metal silicide wires are formed where the solidphase at block 110 of FIG. 2 comprises a metal reagent. Examples ofsolid phase metal reagents includes CaH₂, CaBr₂, Cp₂Ti(CO)₂, V(CO)₆,Cr(CO)₆, Cp₂Cr, Mn₂(CO)₁₀, CpMn(CO)₃, Fe₂(CO)₉, Co₂(CO)₈, CO₄(CO)₁₂,Cp₂Co, Cp₂Ni, Ni(COD)₂, BaH₂, [Ru(CO)₄]_(∞), Os₃(CO)₁₂, Ru₃(CO)₁₂,HFeCo₃(CO)₁₂, Co₂(CO)₈ and H₂FeRu₃(CO)₁₃. Metal reagents at block 110may also be a liquid such as TiCl₄ or Fe(CO)₅.

In another embodiment, the solid phase is a photoactive solid phase.For, example, the photoactive phase can be particulates of a carbonfullerene, carbon nanotubes, quantum dots of CdSe, PbS, Si or Ge,core-shell quantum dots of ZnSe/CdSe or Si/Ge.

At block 120, the solution is ejected through a nozzle in a highelectric field to form a substantially continuous nanofiber through anelectrospinning process. Although expulsion of a single solution thougha single nozzle is described in the embodiment of FIG. 2, other solutionand nozzle configurations can be used with the two and four-componentinks. For example, a coaxial nozzle and dispenser system can be usedthat has an inner annulus and an outer annulus as illustrated in Example16. The polymer, solid phase and a solvent can be combined to form aviscous solution that is the source of fluid flowing through the outerannulus. The selected liquid silane is a second source of fluid that isexpressed through the inner annulus.

For example, the liquid silane flowing through the inner annulus isSi₆H₁₂ cyclohexasilane, Si₆H₁₂ 1-silyl-cyclopentasilane or Si₅H₁₀cyclopentasilane and the solution flowing through the outer annulus ispolyacrylonitrile in dimethylformamide and metal particulates or carbonnanotubes.

In another embodiment, a viscous mixture of a polymer and a solvent isproduced and that mixture is ejected through the outer annulus of thenozzle while simultaneously ejecting a Liquid Silane through an innerannulus of the nozzle. The two streams are directed through a highelectric field to form Core-Shell Fibers. The fibers are transformed tosilicon wires with a carbon outer coating. Many other combinations arealso possible with this coaxial nozzle configuration.

At block 130, the nanofiber that is formed at block 120 from theelectrospinning apparatus is deposited on a conductive substrate. Thesubstrate at block 130 is preferably a metallic foil such as copper foilor aluminum foil. The substrate can also be a conducting carbon fibermatte including a carbon fiber matte constructed of carbon nanotubes. Inone configuration, the substrate includes conductive metallic portionsand insulating portions and the silicon-containing wires that areproduced span the insulating portions of the substrate.

The produced fiber collected at block 130 can be transformed toamorphous silicon or crystalline silicon composites through thermaltreatment or light activation via radical polymerization at block 140.The deposited material can also be collected and transformed at adifferent time and location.

As with the three-component inks, the fibers produced from thefour-component inks are typically transformed using thermal processingat temperatures from 150 to 300° C. to give polysilane-containingmaterials. The deposit can also be transformed using thermal processingat temperatures from 300 to 850° C. to produce amorphoussilicon-containing materials. The deposit can also be transformed usingthermal processing at temperatures from ˜850 to 1414° C. givingcrystalline silicon-containing materials. Some variation in thesetemperature ranges may be seen depending on the nature of the particularsolid phase that is used in the ink. Finally, the deposit can betransformed using laser processing to give crystallinesilicon-containing materials at block 140.

An optional coherent, conductive coating may be applied to thetransformed materials before or after the thermal treatments at block150. The coatings at block 150 can be applied by chemical vapordeposition using argon/acetylene, hydrogen/methane or nitrogen/methaneas precursor gases. The coatings can also be applied by solutiondeposition using a dispersion of conducting carbon milled together withthe silicon containing fraction and a solvent and graphite, C black,graphene, nanotubes or wires as a carbon source.

It can be seen that the coated or non-coated nanofibers or wires thatare produced according to the invention can be used as components ofother composite materials with further processing. This can beillustrated with the production of an electrically-conductingsilicon-composite electrode with a three-component ink or afour-component ink. Referring also to FIG. 3, a method 200 for producingan anode material according to the invention is schematically shown. Atblock 210, nanofibers are produced by electrospinning two orfour-component inks. The fibers are transformed at block 220 by thermalor laser processing. The processed fibers are coated with carbon atblock 230. The carbon coating can be applied with chemical vapordeposition or by solution deposition. Carbon coatings preferablycoatings of graphite, carbon black, graphene, or nanotubes or wires.

At block 240 the coated fibers are combined with an ion conductingbinder to form the body of the electrode. The polymer binder may eitherbe inherently lithium ion conducting, or may become lithium ionconducting by absorbing electrolyte solution. The coated nanofibers aremixed with a binder to give a material structure that can be furthersized and shaped. For example, the binder may include poly(vinylidenefluoride-co-hexafluoropropylene) or sodium carboxymethylcellulose. Somebinders may be volatile and capable of being removed with additionalthermal or laser treatments. Other binders may also be ion orelectrically conductive or have a conductive filler such as a carbonparticulate like KB carbon or graphite.

Electrodes with coated silicon fibers are resistant to cracking from thesizeable volume changes that occur during the lithiation anddelithiation processes during cycling, for example. KB carbon is anelastic carbon and is capable of stretching and compressing duringordinary volume changes and is a preferred conductive binder or fillerat block 240.

In one embodiment, an electrode can be produced by: (a) combining aliquid silane of the formula Si_(n)H_(2n), with a polymer such aspoly(methyl methacrylate), polycarbonate, poly(vinylidenefluoride-co-hexafluoropropylene), sodium carboxymethylcellulose or amixture of polymers and a solvent to form a viscous solution; (b)exposing the viscous solution to a high electric field where continuousfibers are formed and deposited onto a metal foil substrate; (c)transforming the deposit into a material that contains a polysilane, anamorphous silicon and/or a crystalline silicon fraction by thermaltreatment under inert gas at a temperature <400° C.; (d) forming acoherent, ion conductive coating on the external porosity of thesilicon-containing fraction deposited by vapor or solution deposition;and (e) mixing the coated silicon nanofiber material with a binder ofpoly(vinylidene fluoride-co-hexafluoropropylene), sodiumcarboxymethylcellulose and/or KB carbon to form an electrode.

The invention may be better understood with reference to theaccompanying examples, which are intended for purposes of illustrationonly and should not be construed as in any sense limiting the scope ofthe present invention as defined in the claims appended hereto.

EXAMPLE 1

In order to demonstrate the functionality of the electrospinning methodswith different formulations of liquid silane inks, a test reactor wasconstructed. All electrospinning processing and post-depositiontreatments were performed inside inert nitrogen gas gloveboxes withactive oxygen scrubbing unless otherwise specified. After appropriateink formulation, three- and four-component solutions and/or mixtureswere taken up into 1 mL HDPE syringes fitted with blunt-nosed 18 gaugestainless steel needles 2.5 cm in length. The ink-containing syringe andneedle were placed into a syringe pump in horizontal position with aneedle-to-substrate standoff distance of ˜25 cm.

Metallic copper foil pieces (5 cm×5 cm×0.8 mm) were employed as theelectrode substrate in the electrospinning process and were cleanedaccording to the following protocol: rinsing with ˜5 mL isopropanolusing a squirt bottle; rinsing with ˜5 mL 1.5 M hydrochloric acid usinga squirt bottle; rinsing with ˜10 mL deionized water using a squirtbottle; and, drying with a stream of particulate-filtered high-puritynitrogen gas. These substrates were then introduced into anelectrospinning process glovebox.

The substrates were then placed into deposition position by connectingthe metallic foil to an acrylic backdrop using an alligator clip thatalso served to make electrical connection to the ground of the powersupply. A high voltage source (Gamma High Voltage Research Inc. ModelES40P-12W/DDPM) was connected with the positive terminal on the needleand the negative (ground) on the metallic substrate. The syringe pump(Cole Parmer model EW-74900-00) was set to a flow rate of 0.4-0.5 mL/hand allowed to run until the needle was primed with liquid. Once adroplet formed on the outside of the needle, the power source wasadjusted to 15 kV. A collimated halogen light source was used tovisualize the spinning solution/mixture. Immediately after the 15 kV wasapplied, spinning fibers were seen moving from the needle horizontallyto the substrate. The ground plate and needle location were adjusted sothat the fibers were deposited at the center of the foil.

Cyclosilanes such as Si₆H₁₂ and Si₅H₁₀ were prepared and distilled underreduced vacuum yielding 99+% pure colorless liquid (by ¹H NMR). TheSi₅H₁₀ was prepared by reacting Si₅Cl₁₀ with LiAlH₄ and used withoutadditional purification. Inert atmosphere gloveboxes and standardSchlenk techniques were used to preclude the oxidation of liquid silane.This is necessary because Si₆H₁₂ and Si₅H₁₀ are pyrophoric liquids thatburn upon contact with air and are treated as an ignition source andhandled in inert atmosphere. In addition, (SiH₂)_(n) reacts slowly withair and moisture to give amorphous silica.

A three-component ink, Si₆H₁₂/PMMA in toluene, was first used todemonstrate the electrospinning methods and the thermolysis productswere characterized and shown schematically in FIG. 4. A solution of PMMAin toluene was prepared by adding 4.60 g of dry toluene to a flame-driedvial with 0.52 grams of PMMA (Aldrich P/N 182265-500G Lot #07227DH,MW=996,000) mixed via magnetic stirring. The mixture was heated to 75°C. to expedite dissolution of the polymer. Next, 500 μL of thisPMMA/toluene solution was cooled to room temperature and 100 μL ofSi₆H₁₂ was added dropwise giving two colorless immiscible phases withone being rather viscous. After stirring for 15 minutes, the mixtureappeared to be homogeneous with an apparent viscosity that was higherthan either of the immiscible phases indicating the formation of athree-component microemulsion or a single-phase mixture. Electrospinningwas realized as described above using a copper foil as the substrate. Itis noteworthy that this process is also operative when using a lowermolecular weight PMMA polymer (MW=350,000).

After electrospinning, a piece of the sample was cut off with a scissorsand heat treated to ˜350° C. for 30 minutes upon which time a slightlyyellow tint was observed in the deposit.

The microstructure of the heat-treated deposit was then probed using ascanning electron microscope. The microstructure was shown to consist ofwires with diameters from 100 nm to 3 μm. Raman microscopecharacterization (Horiba Jobin Yvon, LabRAM ARAMIS, 532 nm illumination)of the product confirmed the existence of amorphous silicon phase giventhe characteristic broad band at 485 cm⁻¹. Interestingly, the Ramanlaser can transform the a-Si wires into crystalline Si as evidenced by aband at 513 cm⁻¹ that was observed after the laser beam was focused to˜100 kW/cm². Optical micrographs of the electrospun deposit subjected tothe higher power density show clear signs of melting and densificationin the wire.

The produced electrospun nanowire materials were collected and testedfor electrode performance by using the materials to make anodes inelectrochemical cells. Before assembly in pouch cells, the a-Si wireswere exposed to air and loaded into a chemical vapor chamber where athin conducting carbon layer ˜10 nm thick was deposited. Afterwards, theC-coated a-Si wires were moved into a second inert atmosphereargon-filled glove box (H₂O and O₂<1 ppm). Lithium metal/a-Si wirehalf-cells were fabricated using Celgard-2300 as the separator and 1 MLiP F₆ in ethylene carbonate:diethyl carbonate 1:1 as the electrolytewith a mass loading of 4 mg/cm². Electrochemical testing was performedby cycling between 0.02 and 1.50 V at 100 mA/g using an Arbin modelB2000 tester. The charge/discharge data for a half-cell comprised oflithium metal and chemical vapor deposition carbon-coated a-Si nanowiresprepared according to Example 1 was recorded and demonstrated comparablecharacteristics over 30 cycles.

EXAMPLE 2

Electrospinning of a three-component ink, Si₆H₁₂/PMMA using the solventdichloromethane (DCM), was conducted to demonstrate an alternativesolvent and to characterize performance of the resulting material as anelectrode. A solution of PMMA in DCM was prepared by adding 18.0 mL ofdry DCM to a flame-dried vial with 2.681 g of PMMA mixed via magneticstirring at 500 RPM for 3 h. Next, 8.220 g of this PMMA/DCM solution,858 μL of DCM and 418 μL of Si₆H₁₂ were added dropwise whilemagnetically stirring to give a mixture of two immiscible liquids. Afterstirring for 15 minutes, the mixture appeared to be homogeneous with anapparent viscosity that was higher than either of the immiscible phasesindicating the formation of a three-component microemulsion or asingle-phase mixture. Electrospinning was realized as described aboveusing a copper foil as the substrate.

Immediately after electrospinning each 1 mL aliquot, the deposited wireswere scraped off of the copper foil and placed inside a flame-driedvial. The vials containing the samples were then heated on a ceramichotplate with an aluminum shroud to 550° C. with a ramp rate no slowerthan 16° C./minute, and held for 1 h. The microstructure of theheat-treated deposit was probed using high-resolution scanning electronmicroscope and shown to consist of porous wires and agglomerates withprimary particle size ˜150 nm in diameter. Raman microscopecharacterization of the product confirmed the existence of amorphoussilicon phase given the characteristic broad band at 485 cm⁻¹. The Ramanlaser could also transform the a-Si wires into crystalline Si asevidenced by a band at 516 cm⁻¹ that was observed after the laser beamwas focused to ˜100 kW/cm².

Optical micrographs of the electrospun deposit subjected to the higherpower density showed clear signs of melting and densification in thewire. An 80 mg sample of the heated sample was sent to GalbraithLaboratories (Knoxville, Tenn.) for ICP-OES and combustion analysiswhere duplicate analyses showed 83.6 wt % silicon and 6.6 wt % carbon.

The produced nanowire materials were then used to make anodes inelectrochemical cells. Before assembly in pouch cells, the a-Si wireswere exposed to air and loaded into a chemical vapor chamber where athin conducting carbon layer ˜10 nm thick was deposited. Afterwards, theC-coated a-Si wires were moved into a second inert atmosphereargon-filled glove box (H₂O and O₂<1 ppm). Lithium metal/a-Si wirehalf-cells were fabricated using Celgard-2300 as the separator and 1 MLiP F₆ in ethylene carbonate:diethyl carbonate 1:1 as the electrolytewith a mass loading of 4 mg/cm². Electrochemical testing was performedby cycling between 0.02 and 1.50 V at 100 mA/g using an Arbin modelB2000 tester. Charge/discharge data for a half-cell comprised of lithiummetal and chemical vapor deposition carbon-coated a-Si nanowires wasobtained. Specific capacity data showed an initial capacity of 3400mAh/g, a 2nd cycle capacity of 2693 mAh/g with a fade of 16.6% after 21cycles.

EXAMPLE 3

The product of a second three-component ink, Si₅H₁₀/PMMA in DCM with apost deposit treatment of 550° C. for 60 minutes and laser exposure wascharacterized. A 10 wt % polymer solution was prepared by adding driedand nitrogen-sparged DCM into a flame-dried glass vial with PMMAdissolved by stirring for ˜12 h. At that time, 45 μL of Si₅H₁₀ was addedto the solution using a micropipette and this mixture was stirred for 10minutes using a PTFE-coated magnetic stir bar. The copper foil substratewas cleaned and moved into the electrospinning glovebox before beingmounted and connected to the apparatus. Electrospinning was performedwith a 20 cm stand-off distance, a 12 kV excitation, 0.5 mL/h ink flowrate and a total solution volume of ˜75 μL was dispensed.

Post thermal treatment of the electrospun sample on copper foil wasconducted in a nitrogen ambient (<1 ppm O₂ and H₂O). The sample wasplaced on a room temperature ceramic hotplate, and covered with analuminum heat shield to improve temperature uniformity. The hotplate wasramped to 550° C. no slower than 30° C./minute and held at nominally550° C. for one hour after which time, the sample was removed from thehotplate and placed on a room temperature aluminum plate and allowed toquickly cool to ambient.

Optical micrographs of the electrospun collected sample depicted wiresthat were ˜1 μm in diameter. Raman characterization of these wiresshowed the existence of crystalline silicon after melting with the Ramanlaser.

EXAMPLE 4

The product of a three-component ink Si₆H₁₂/QPAC100 in toluene wascharacterized by two different post deposit treatments: heating at 350°C. for 20 minutes; or 355 nm laser exposure followed by heating at 350°C. for 20 minutes. The latter of these two processes is shownschematically in FIG. 5.

A polymer solution was prepared by placing 1.06 g of dried toluene intoa flame-dried vial and adding 120 mg QPAC100 while stirring with aPTFE-coated magnetic stir bar for 2.5 h at 500 rpm. At this time, 50 μLSi₆H₁₂ was added via pipette and a slight immiscibility was noted. Themixture was stirred for ˜40 h yielding a homogeneous mixture. The copperfoil substrate was cleaned and moved into the electrospinning gloveboxbefore being mounted and connected to the apparatus. Prior toelectrospinning, the substrate was heat treated for one minute at 350°C. to desorb any trace water. Electrospinning was performed with a 30 cmstand-off distance, 0.5 mL/h ink flow rate and a 10 kV excitation.

After spinning for one hour, the sample was removed and cut into pieceswith one being subjected to thermal treatment at 350° C. for 20 minutes.Interestingly, no wire like deposit was noted by optical microscopyafter this thermal treatment. Scanning electron microscopycharacterization showed dark areas that originated from the electrospundeposits with Raman characterization indicating the presence of a-Si onthe substrate.

A description of this phenomenon can be envisioned by consideration ofthe thermal properties of each of the constituents of thisthree-component ink. Firstly, Si₆H₁₂ shows that evaporation begins ataround 225° C. with some polymerization that gives 32.9% residual massafter heating to 350° C. Secondly, QPAC100 begins to thermalize around150° C. with 50% mass loss observed at 270° C. and less than 1% residueat 350° C. Therefore, when the electrospun wire formed by thethree-component Si₆H₁₂/QPAC100 ink was thermally-treated, the polymercomponent volatized prior to the formation of a structurally stablepoly(dihydrosilane). As the Si₆H₁₂ fraction was yet unpolymerized,nanosized Si films appeared as shadows of the original wires.

After spinning for one hour, the second sample was cut into pieces andone was placed in an air-tight container and transferred into a gloveboxthat contained a beam from a HIPPO laser (355 nm illumination, SpectraPhysics Inc.). Variable laser powers of 500 mW, 1 W, 2 W, 3 W, and 4 Wfor 1 minute and also 500 mW and 4 W for 5 minutes transformed theSi₆H₁₂ into polysilane as evidenced by the appearance of yellow/browndiscolorations for incident areas of the Si₆H₁₂/QPAC100 deposit. Afterthis photolysis step, the (SiH₂)_(n)/QPAC100 sample was placed on a roomtemperature hotplate and heated to 341° C. for a total of 20 minutes.The a-Si wires that were formed were characterized by high-resolutionscanning electron microscopy and shown to possess significant porosity.Raman characterization of the product confirmed the existence ofamorphous silicon phase that was melted by focusing the Raman laser.

EXAMPLE 5

The electrospun fibers of a four-component ink PMMA/Si₆H₁₂/Co₂(CO)₁₀ inDCM and the resulting thermolysis products were characterized. Asolution of PMMA in toluene was prepared by adding 10.38 mL of drytoluene to a flame-dried vial with 980 mg of PMMA mixed via magneticstirring. 50 mg of a cobalt/silicon solution and 1 mL of thePMMA/toluene solution were mixed in a 4 mL flame-dried vial. Afterstirring for 15 minutes, the mixture appeared to be homogeneous.Electrospinning was realized as described above using a copper foil asthe substrate.

After electrospinning, a piece of the sample was cut off with a scissorsand rapidly thermal annealed to ˜600° C. using an IR lamp. A piece ofthis sample was adhered to a glass slide with silver contacts which weredeposited with a wood toothpick using fast-drying silver paint.Resistance across the two silver contacts was measured using a two-pointmethod with the Agilent B1500A semiconductor analyzer using I-Vanalysis. Resistivity values were obtained by manually approximating theamount of wires which were connecting between the electrodes andapproximating the length between the electrodes (2 mm) and approximatingthe wire diameter (3-4 μm). The resistance was measured and resistivitycalculated to be 4×10⁴ Ω-m.

The microstructure of the heat-treated wires was probed using a highresolution scanning electron microscope and shown to consist of wireswith diameters from 1 to 3 μm. EDS mapping confirms the presence ofcobalt and silicon within the wires. The non-polymer components of thisfour-component electrospinning ink (i.e., Si₆H₁₂ and Co₂(CO)₈) havepreviously been reported as reagents for forming silicon-cobalt films.

EXAMPLE 6

Another four-component ink, PMMA/Si₆H₁₂/CdSe in DCM and its thermolysisproducts were characterized. A 10 wt % solution of PMMA in dried andnitrogen-sparged DCM was mixed for ˜12 h and then 0.931 g of thissolution was added to a flame-dried glass vial. To that solution, 46 μLof Si₆H₁₂ and 47 μL of CdSe quantum dots in toluene (Lumidot® 480 nmexcitation, 5 mg/mL in toluene, Sigma Aldrich P/N662356) were stirredfor 10 minutes using a Teflon-coated magnetic stir bar. Electrospinningemployed a copper substrate and was performed as described above.

Post-deposition treatment of electrospun deposit was performed in anitrogen ambient (<1 ppm O₂ and H₂O). The sample was placed on a roomtemperature ceramic hotplate, and covered with an aluminum heat shieldto improve temperature uniformity. The hotplate was ramped to 550° C. noslower than 30° C./minute and held at nominally 550° C. for one hour.Thereafter, the sample was removed from the hotplate and placed on aroom temperature aluminum plate and allowed to quickly cool to ambienttemperature. The sample was then analyzed by Raman spectroscopy and thecharacteristic peak for crystalline silicon was noted after treatmentwith the Raman laser as shown in FIG. 6.

EXAMPLE 7

A third four-component ink, PMMA/Si₆H₁₂/Carbon Black in DCM, and itsthermolysis products were characterized. A suspension of carbon black(Cabot Industries, Black Pearls 2000) was prepared by mixing 52 mg ofthe carbon black with 1 mL of dried and nitrogen-sparged DCM in aflame-dried glass vial and sonicated for 30 minutes.

In a second flame-dried glass vial was placed 0.963 g of a 10 wt %solution of PMMA in dried and nitrogen-sparged DCM was mixed for ˜12 h.To that solution, 48 μL of Si₆H₁₂ and 12 mg of the dried sonicatedcarbon black suspension were stirred for 10 minutes using aTeflon-coated magnetic stir bar. Electrospinning employed a coppersubstrate and was performed as described previously.

Post-deposition treatment of the electrospun deposit was performed in anitrogen ambient (<1 ppm O₂ and H₂O) atmosphere. The sample was thenplaced on a room temperature ceramic hotplate, and covered with analuminum heat shield to improve temperature uniformity. The hotplate wasramped to 550° C. no slower than 30° C./minute and held at nominally550° C. for one hour after which time, the sample was removed from thehotplate and placed on a room temperature aluminum plate and allowed toquickly cool to ambient temperature. The sample was analyzed by Ramanand the characteristic peak for crystalline silicon was noted aftertreatment with the Raman laser as shown in FIG. 6.

EXAMPLE 8

For comparison, a fourth four-component ink, PMMA/Si₆H₁₂/graphite inDCM, and its thermolysis products were characterized. A suspension ofgraphite (Asbury Carbon, grade 4934) was prepared by mixing 52 mg of thegraphite with 1 mL of dried and nitrogen-sparged DCM in a flame-driedglass vial and sonicated for 30 minutes. A 10 wt % solution of PMMA indried and nitrogen sparged DCM was mixed for ˜12 h and 0.942 g of thissolution was added to a flame-dried glass vial. To that solution, 47 μLof Si₆H₁₂ and 47 μL of the sonicated graphite suspension were stirredfor 10 minutes using a Teflon-coated magnetic stir bar. Electrospinningemployed a copper substrate and was performed as described above.

Post-deposition treatment of the electrospun deposit was performed in anitrogen ambient (<1 ppm O₂ and H₂O). The sample was placed on a roomtemperature ceramic hotplate, and covered with an aluminum heat shieldto reduce temperature inhomogeneity. The hotplate was ramped to 550° C.no slower than 30° C./minute and held at nominally 550° C. for one hourafter which time, the sample was removed from the hotplate and placed ona room temperature aluminum plate and allowed to quickly cool toambient. The sample was analyzed by Raman and the characteristic peakfor crystalline silicon was noted after treatment with the Raman laseras shown in FIG. 6.

EXAMPLE 9

The product of a fifth four-component ink, PMMA/Si₆H₁₂/Ag in DCM wascharacterized for comparison. In this illustration, a suspension ofsilver nanoparticles (<100 nm diameter, Sigma Alrich P/N 576832) wasprepared by mixing 35 mg of the silver nanopowder with 700 μL of driedand nitrogen-sparged DCM in a flame-dried glass vial and sonicated for30 minutes. A 10 wt % solution of PMMA in dried and nitrogen-sparged DCMwas mixed for ˜12 h at which time 0.923 g of this solution was added toa flame-dried glass vial. To that solution, 46 μL of Si₆H₁₂ and 46 μL ofthe sonicated silver nanoparticle suspension were stirred for 10 minutesusing a Teflon coated magnetic stir bar. Electrospinning employed acopper substrate and was performed as described above.

Post-deposition treatment of the electrospun deposit was performed in anitrogen ambient (<1 ppm O₂ and H₂O). The sample was placed on a roomtemperature ceramic hotplate, and covered with an aluminum heat shieldto improve temperature uniformity. The hotplate was ramped to 550° C. noslower than 30° C./minute and held at nominally 550° C. for one hourafter which time, the sample was removed from the hotplate and placed ona room temperature aluminum plate and allowed to quickly cool toambient. The sample was analyzed by Raman and the characteristic peakfor crystalline silicon was observed after treatment with the Ramanlaser as shown in FIG. 6.

EXAMPLE 10

A sixth four-component ink, PMMA/Si₆H₁₂/AIP in DCM, was characterized tofurther demonstrate the breadth of the methods. A 10 wt % solution ofPMMA in dried and nitrogen-sparged DCM was mixed for ˜12 h after whichtime 0.949 g of this solution was added to a flame-dried glass vial. Tothat solution, 47 μL of Si₆H₁₂ and 47 μL of an amphiphilic invertiblepolymer (AIP) (synthesized from poly(ethylene glycol) (PEG) andaliphatic dicarboxylic acids) were stirred for 10 minutes using a Tefloncoated magnetic stir bar. Electrospinning employed a copper substrateand was performed as described above.

Post-deposition treatment of electrospun deposit was performed in anitrogen ambient (<1 ppm O₂ and H₂O). The sample was placed on a roomtemperature ceramic hotplate, and covered with an aluminum heat shieldto improve temperature uniformity. The hotplate was ramped to 550° C. noslower than 30° C./minute and held at nominally 550° C. for one hourafter which time, the sample was removed from the hotplate and placed ona room temperature aluminum plate and allowed to quickly cool toambient. The sample was analyzed by Raman and the characteristic peakfor crystalline silicon was noted after treatment with the Raman laseras shown in FIG. 6.

EXAMPLE 11

The products of a seventh four-component ink, PMMA/Si₆H₁₂/BBr₃ in DCMwere also characterized. A 10 wt % solution of PMMA in dried andnitrogen-sparged DCM was mixed for ˜12 h and 0.931 g of this solutionwas added to a flame-dried glass vial. To that solution, 46 μL of Si₆H₁₂and 1.5 μL of BBr₃ (>99.99% pure, Sigma Aldrich P/N 230367) were addedand stirred for 10 minutes using a Teflon-coated magnetic stir bar.Electrospinning employed a copper substrate and was performed asdescribed above.

Post-deposition treatment of the electrospun deposit was performed in anitrogen ambient (<1 ppm O₂ and H₂O). The sample was placed on a roomtemperature ceramic hotplate, and covered with an aluminum heat shieldto improve temperature uniformity. The hotplate was ramped to 550° C. noslower than 30° C./minute and held at nominally 550° C. for one hourafter which time, the sample was removed from the hotplate and placed ona room temperature aluminum plate and allowed to quickly cool toambient. The sample was analyzed by Raman and the characteristic peakfor crystalline silicon was noted after treatment with the Raman laseras shown in FIG. 6.

EXAMPLE 12

The electrospin products of an eighth four-component ink,PMMA/Si₆H₁₂/PBr₃ in DCM, were characterized for comparison. A 10 wt %solution of PMMA in dried and nitrogen-sparged DCM was mixed for ˜12 hat which time 1.522 g of this solution was added to a flame-dried glassvial. To that solution, 75 μL of Si₆H₁₂ and 2.3 μL of PBr₃ (>99.99%pure, Sigma Aldrich P/N 288462) were stirred for 10 minutes using aTeflon coated magnetic stir bar. Electrospinning employed a coppersubstrate and was performed as described above.

Post-deposition treatment of the electrospun deposit was performed in anitrogen ambient (<1 ppm O₂ and H₂O). The sample was placed on a roomtemperature ceramic hotplate, and covered with an aluminum heat shieldto improve temperature uniformity. The hotplate was ramped to 550° C. noslower than 30° C./minute and held at nominally 550° C. for one hourafter which time, the sample was removed from the hotplate and placed ona room temperature aluminum plate and allowed to quickly cool toambient. The sample was analyzed by Raman and the characteristic peakfor crystalline silicon was noted after treatment with the Raman laseras shown in FIG. 6.

It can be seen that many different novel two or three component inks andsilicon based nanowires and materials can be commercially produced inelectrospinning reactors, and the feasibility of producing efficientnanowire electrodes was demonstrated.

EXAMPLE 13

The products of a ninth four-component ink, PMMA/Si₆H₁₂/CNTs in DCM werealso characterized. A 10 wt % solution of PMMA in dried andnitrogen-sparged DCM was mixed for ˜12 h at which time 1.960 g of thissolution was added to a flame-dried glass vial that contained 4.04 mg ofcarbon nanotubes (Sigma Aldrich P/N 704148). To that solution, 98 μL ofSi₆H₁₂ were added and stirred for 10 minutes using a Teflon coatedmagnetic stir bar. Electrospinning employed a copper substrate and wasperformed as described above.

Post-deposition treatment of the electrospun deposit was performed in anitrogen ambient (<1 ppm O₂ and H₂O). After spinning, the sample was cutinto pieces and one was placed in an air-tight container and transferredinto a glovebox that contained a beam from a HIPPO laser (355 nmillumination, Spectra Physics Inc.). A laser power of 750 mW with a 1cm² spot size was used to scan across the entire sample at a rate of 5mm/s. After this photolysis step, the (SiH₂)_(n)/PMMA sample was placedon a room temperature hotplate and heated to 350° C. at a ramp rate of50° C./10 minutes. The sample was analyzed by Raman and thecharacteristic peak for crystalline silicon, as well as the D and Gbands of the carbon nanotubes were noted after treatment with the Ramanlaser.

EXAMPLE 14

The spin coating of thin films using a three-component ink, Si₆H₁₂/PMMAin DCM was demonstrated and compared with nanofibers produced by aconventional nozzle. A 10 wt % solution of PMMA in dried andnitrogen-sparged DCM was mixed for ˜12 h at which time 0.862 g of thissolution was transferred to a flame-dried glass vial. To that solution,43 μL of Si₆H₁₂ was added and then stirred for 10 minutes using aTeflon-coated magnetic stir bar. The solution volume was then doubled bydiluting with additional DCM.

Fused silica and quartz (1 cm×1 cm×1 mm) were employed as substrate inthe spin coating process and were cleaned according to the followingprotocol: Liquinox™ detergent cleaning by rubbing for 30 sec with alatex glove; rinsing in a stream of hot water for 15 seconds; rinsingwith ˜10 mL deionized water using a squirt bottle; rinsing with ˜10 mLacetone using a squirt bottle; rinsing with ˜10 mL isopropanol using asquirt bottle; and, drying with the flame of a propane torch. For thespin-coating procedure, 30 μL of the Si₆H₁₂/PMMA sample was dispensedonto a quartz substrate while spinning at 3000 RPM and under UVirradiation from a Hg(Xe) arc lamp (Newport Corp, lamp model 66142,power density ˜50 mW/cm²) with a dichroic mirror used to filter theinfrared photons.

Thermal treatment of samples deposited on fused silica and quartz wasconducted in a nitrogen ambient (<1 ppm O₂ and H₂O). The samples wereplaced on a room temperature aluminum hotplate and covered with analuminum heat shield to improve temperature uniformity. The hotplate wasramped to 350° C. at 250° C./h at which time the thermal treatment wasquenched by removing the sample from the hotplate to an aluminum plateat ambient temperature. Raman characterization of these films showed theexistence of crystalline silicon after melting with the Raman laser.

EXAMPLE 15

Spin coating of thin films using a four-component ink, Si₆H₁₂/PMMA/Ag inDCM was conducted to illustrate fiber formation from a thin film forcomparison with other fiber producing methods. A mixture of silvernanoparticles (<100 nm diameter, Sigma Alrich P/N 576832) was preparedby mixing 35 mg of the silver nanopowder with 700 μL of dried andnitrogen-sparged DCM in a flame-dried glass vial. The vial was placed inan ultrasonic bath and treated with sonics for 30 minutes. A 10 wt %solution of PMMA in dried and nitrogen-sparged DCM was mixed for ˜12 hat which time 0.923 g of this solution was transferred to a flame-driedglass vial. To this PMMA solution was added 46 μL of Si₆H₁₂ and 46 μL ofthe sonicated Ag/DCM mixture and the entire contents were stirred for 10minutes using a Teflon-coated magnetic stir bar. The solution volume wasthen doubled by diluting with additional DCM.

Fused silica and quartz substrates (1 cm×1 cm×1 mm) were cleaned asdescribed above. Thin films were prepared by spun-coating as describedabove using 30 μL of the four-component ink (Si₆H₁₂/PMMA/Ag). Afterspin-coating, thermal treatment of samples deposited on fused silica andquartz was conducted in a nitrogen ambient (<1 ppm O₂ and H₂O). Thesamples were placed on a room temperature aluminum hotplate and coveredwith an aluminum heat shield to improve temperature uniformity. Thehotplate was ramped to 350° C. at 250° C./h at which time the thermaltreatment was quenched by removing the sample from the hotplate to analuminum plate at ambient temperature. Raman characterization of thesefilms showed the existence of crystalline silicon after melting with theRaman laser.

EXAMPLE 16

In some instances a liquid that serves as a solvent for the polymer mayreact with Si₆H₁₂. A coaxial electrospinning approach can be employed tocircumvent the deleterious interaction of Si₆H₁₂ with some solvents. Theproduct formed by coaxial electrospinning where neat Si₆H₁₂ and apoly(acrylonitrile) (PAN) in dimethylformamide (DMF) solution wereexpelled from the inner and outer tubes, respectively was heat treatedto 350° C. in nitrogen ambient for one hour, in air at 350° C. for onehour, and in nitrogen at 800° C. for one hour.

The PAN in DMF solution was prepared by placing 2.465 g of dried DCMinto a flame-dried vial and adding a total of 548 mg PAN while stirringwith a PTFE-coated magnetic stir bar for 24 h at 500 rpm. A 7.62 cm×7.62cm×0.762 mm copper foil substrate was cleaned as previously mentionedand moved into the electrospinning glovebox before being mounted andconnected to the apparatus. Electrospinning was performed with a 20 cmstand-off distance, 0.5 mL/h flow rate of both the inner and outerfluids and a 10 to 19 kV excitation.

After spinning for one hour, the sample was removed and subjected tothermal treatment at 350° C. for one hour on a hotplate in nitrogenambient (<1 ppm O₂ and H₂O) with a ramp rate of 200° C./h, followed bytube furnace treatment in air at 350° C. for one hour and nitrogenambient at 800° C. for one hour. Optical microscopy of the annealedcoaxial electrospun sample confirms the presence of wire-like depositswith diameter ˜1 μm. Raman analysis of this same sample shows thepresence of silicon, as evidenced by a ˜480 cm⁻¹ and 520 cm⁻¹ bandscorresponding to a-Si and c-Si, respectively.

A description of this phenomenon can be envisioned by consideration ofthe thermal properties of each of the constituents of thisthree-component ink. Firstly, Si₆H₁₂ shows that evaporation begins ataround 225° C. with some polymerization that gives 32.9% residual massafter heating to 350° C. Secondly, PAN crosslinks around 350° C. in airand thermalizes to carbon around 800° C. in nitrogen. Therefore, whenthe coaxial electrospun wires formed from the three-component Si₆H₁₂/PANink were thermally-treated, the silicon component converts to a-Siand/or c-Si and the polymer component carbonizes to form structurallystable and conductive carbon.

From the discussion above it will be appreciated that the invention canbe embodied in various ways, including the following:

1. A method for synthesizing silicon nanofibers, comprising combining aliquid silane, a polymer and a solvent to form a viscous solution;passing a stream of viscous solution through a high electric field toform fibers; depositing the formed fibers onto a substrate; and thentransforming the deposited fibers.

2. The method of embodiment 1, wherein the liquid silane is acyclosilane of the formula Si_(n)H_(2n) selected from the group ofcyclosilanes consisting essentially of cyclopentasilane, cyclohexasilaneand 1-silylcyclopentasilane.

3. The method of embodiment 1, wherein the liquid silane is a linear orbranched silane of the formula Si_(n)H_(2n+2)

4. The method of embodiment 1, wherein the polymer is selected from thegroup of polymers consisting essentially of poly(methyl methacrylate), apolycarbonate, poly(vinylidene fluoride-co-hexafluoropropylene), andpolyvinyl butryal.

5. The method of embodiment 1, wherein the solvent is selected from thegroup of solvents consisting essentially of toluene, xylene,cyclooctane, 1,2,4-trichlorobenzene, dichloromethane and mixturesthereof.

6. The method of embodiment 1, wherein the substrate is selected fromthe group of substrates consisting essentially of a carbon fiber matte,a metal foil, and a mandrel.

7. The method of embodiment 1, wherein the deposited fibers aretransformed using thermal processing at temperatures from 150° C. to300° C. to produce polysilane-containing materials.

8. The method of embodiment 1, wherein the deposited fibers aretransformed using thermal processing at temperatures from 300° C. to850° C. to produce amorphous silicon-containing materials or wherein thetransformed fiber contains polymer and amorphous silicon.

9. The method of embodiment 1, wherein the deposited fibers aretransformed using thermal processing at temperatures from 850° C. to1414° C. to produce crystalline silicon-containing materials.

10. The method of embodiment 1, wherein the deposited fibers aretransformed using laser processing to give crystallinesilicon-containing materials.

11. A method for synthesizing silicon nanofibers, comprising: combininga liquid silane, a polymer, a solid phase and a solvent to form aviscous solution; passing a stream of viscous solution through a highelectric field to form fibers; depositing the formed fibers onto asubstrate; and then transforming the deposited fibers.

12. The method of embodiment 11, wherein the liquid silane is acyclosilane of the formula Si_(n)H_(2n) selected from the group ofcyclosilanes consisting essentially of cyclopentasilane, cyclohexasilaneand 1-silylcyclopentasilane.

13. The method of embodiment 11, wherein the liquid silane is a linearor branched cyclosilane of the formula Si_(n)H_(2n+2).

14. The method of embodiment 11, wherein the solid phase is a metallicparticle selected from the group of metal particles consistingessentially of metallic particles of Al, Au, Ag, Cu, In—Sn—O,fluorine-doped tin oxide and carbon black.

15. The method of embodiment 11, wherein the solid phase is asemiconducting particle selected from the group of semiconductingparticles consisting essentially of carbon nanotubes, silicon nanowires,polydihydrosilane (Si_(n)H₂)_(n), CdSe, CdTe, PbS, PbSe, ZnO and Si.

16. The method of embodiment 11, wherein the solid phase is a metalreagent selected from the group of metal reagents consisting essentiallyof CaH₂, CaBr₂, Cp₂Ti(CO)₂, TiCl₄, V(CO)₆, Cr(CO)₆, Cp₂Cr, Mn₂(CO)₁₀,CpMn(CO)₃, Fe(CO)₅, Fe₂(CO)₉, Co₂(CO)₈, CO₄(CO)₁₂, Cp₂Co, Cp₂Ni,Ni(COD)₂, BaH₂, [Ru(CO)₄]_(∞), Os₃(CO)₁₂, Ru₃(CO)₁₂, HFeCo₃(CO)₁₂, andH₂FeRu₃(CO)₁₃.

17. The method of embodiment 11, wherein the solid phase is aphotoactive particle selected from the group of photoactive particlesconsisting essentially of a carbon fullerene, a quantum dot of CdSe,PbS, Si or Ge, and a core-shell quantum dot of ZnSe/CdSe or Si/Ge.

18. The method of embodiment 11, further comprising coating thetransformed fibers with a coherent, conductive coating.

19. The method of embodiment 11, wherein the coating is a coatingselected from the group of coatings consisting essentially of graphite,carbon black, KB Carbon, carbon nanotubes and graphene.

20. A method of making silicon-containing composite wires comprising:combining a polymer and a solvent to form a viscous solution; flowingliquid silane through the inner annulus of a coaxial delivery tube whileflowing the viscous polymer mixture through the outer annulus; exposingthe viscous solution to a high electric field where continuous fibersare formed and deposited onto a substrate; and transforming thedeposited fibers into a composite material that contains on the inside apolysilane, an amorphous silicon and/or a crystalline silicon fractionand on the outside a carbon coating.

21. The method of embodiment 20, wherein the liquid silane flowingthrough the inner annulus is selected from the group of cyclosilanesconsisting essentially of Si₆H₁₂ cyclohexasilane, Si₆H₁₂1-silyl-cyclopentasilane and Si₅H₁₀ cyclopentasilane.

22. The method of embodiment 20, wherein the liquid silane flowingthrough the inner annulus is selected from the group of linear andbranched silanes consisting essentially of Si_(n)H_(2n+2).

23. The method of embodiment 20, wherein the solution flowing throughthe outer annulus is polyacrylonitrile in dimethylformamide.

24. A method for making silicon-containing battery electrode composite,comprising: combining a liquid silane of the formula Si_(n)H_(2n), witha polymer and a solvent to form a viscous solution; expelling theviscous solution through a high electric field wherein continuous fibersare formed and deposited onto a metal foil substrate; transforming thedeposited fibers by thermal treatment under inert gas; forming acoherent, ion conductive coating on the transformed fibers; and mixingthe coated silicon nanofibers with a binder and KB carbon to form anelectrode.

25. An electrospinning ink, comprising a liquid silane of the formulaSi_(n)H_(2n); a polymer; and a solvent.

26. An electrospinning ink, comprising a liquid silane of the formulaSi_(n)H_(2n); a polymer; a solid phase; and a solvent.

27. The electrospinning ink of embodiment 26, wherein the solid phase isa metallic particle selected from the group of metal particlesconsisting essentially of spherical metallic particles of Al, Au, Ag,Cu, In—Sn—O, fluorine-doped tin oxide and carbon black.

28. The electrospinning ink of embodiment 26, wherein the solid phase isa semiconducting particle selected from the group of semiconductingparticles consisting essentially of carbon nanotubes, silicon nanowires,polydihydrosilane (Si_(n)H₂)_(n), CdSe, CdTe, PbS, PbSe, ZnO and Si.

29. The electrospinning ink of embodiment 26, wherein the solid phase isa metal reagent selected from the group of metal reagents consistingessentially of CaH₂, CaBr₂, Cp₂Ti(CO)₂, TiCl₄, V(CO)₆, Cr(CO)₆, Cp₂Cr,Mn₂(CO)₁₀, CpMn(CO)₃, Fe(CO)₅, Fe₂(CO)₉, Co₂(CO)₈, CO₄(CO)₁₂, Cp₂Co,Cp₂Ni, Ni(COD)₂, BaH₂, [Ru(CO)₄]_(∞), Os₃(CO)₁₂, Ru₃(CO)₁₂,HFeCo₃(CO)₁₂, and H₂FeRu₃(CO)₁₃.

30. The electrospinning ink of embodiment 26, wherein the solid phase isa photoactive particle selected from the group of photoactive particlesconsisting essentially of a carbon fullerene, a quantum dot of CdSe,PbS, Si or Ge, and a core-shell quantum dot of ZnSe/CdSe or Si/Ge.

31. The electrospinning ink of embodiment 26, wherein the cyclosilane isa branched cyclosilane of the formula Si_(n)H_(2n+2)

Although the description above contains many details, these should notbe construed as limiting the scope of the invention but as merelyproviding illustrations of some of the presently preferred embodimentsof this invention. Therefore, it will be appreciated that the scope ofthe present invention fully encompasses other embodiments which maybecome obvious to those skilled in the art, and that the scope of thepresent invention is accordingly to be limited by nothing other than theappended claims, in which reference to an element in the singular is notintended to mean “one and only one” unless explicitly so stated, butrather “one or more.” All structural, chemical, and functionalequivalents to the elements of the above-described preferred embodimentthat are known to those of ordinary skill in the art are expresslyincorporated herein by reference and are intended to be encompassed bythe present claims. Moreover, it is not necessary for a device or methodto address each and every problem sought to be solved by the presentinvention, for it to be encompassed by the present claims. Furthermore,no element, component, or method step in the present disclosure isintended to be dedicated to the public regardless of whether theelement, component, or method step is explicitly recited in the claims.No claim element herein is to be construed as a “means plus function”element unless the element is expressly recited using the phrase “meansfor”. No claim element herein is to be construed as a “step plusfunction” element unless the element is expressly recited using thephrase “step for”.

What is claimed is:
 1. An electrospinning ink, comprising: a liquidsilane of the formula Si_(n)H_(2n); a polymer; and a solvent.
 2. Anelectrospinning ink, comprising: a liquid silane of the formulaSi_(n)H_(2n); a polymer; a solid phase; and a solvent.
 3. Anelectrospinning ink as recited in claim 2, wherein said solid phase is ametallic particle selected from the group of metal particles consistingessentially of spherical metallic particles of Al, Au, Ag, Cu, In—Sn—O,fluorine-doped tin oxide and carbon black.
 4. An electrospinning ink asrecited in claim 2, wherein said solid phase is a semiconductingparticle selected from the group of semiconducting particles consistingessentially of carbon nanotubes, silicon nanowires, polydihydrosilane(Si_(n)H₂)_(n), CdSe, CdTe, PbS, PbSe, ZnO and Si.
 5. An electrospinningink as recited in claim 2, wherein said solid phase is a metal reagentselected from the group of metal reagents consisting essentially ofCaH₂, CaBr₂, Cp₂Ti(CO)₂, TiCl₄, V(CO)₆, Cr(CO)₆, Cp₂Cr, Mn₂(CO)₁₀,CpMn(CO)₃, Fe(CO)₅, Fe₂(CO)₉, Co₂(CO)₅, CO₄(CO)₁₂, Cp₂Co, Cp₂Ni,Ni(COD)₂, BaH₂, [Ru(CO)₄]_(∞), Os₃(CO)₁₂, Ru₃(CO)₁₂, HFeCo₃(CO)₁₂, andH₂FeRu₃(CO)₁₃.
 6. An electrospinning ink as recited in claim 2, whereinsaid solid phase is a photoactive particle selected from the group ofphotoactive particles consisting essentially of a carbon fullerene, aquantum dot of CdSe, PbS, Si or Ge, and a core-shell quantum dot ofZnSe/CdSe or Si/Ge.
 7. An electrospinning ink as recited in claim 2,wherein said liquid silane is a linear or branched cyclosilane of theformula Si_(n)H_(2n+2).